The rack-and-pinion steering apparatus popularly employed for an automobile is provided with a pinion shaft having pinion teeth provided on a circumferential surface thereof connected to a steering member such as a steering wheel, and a rack shaft having rack teeth provided over an appropriate length in a middle region of an outer surface thereof and disposed so as to extend in a left and right direction of the vehicle body, and converts the rotation of the pinion shaft according to operation of the steering member by the driver into an axial movement of the rack shaft, thus to steer the wheels for steering (generally, left and right front wheels) connected to the left and right ends of the rack shaft via respective tie rods.
Regarding such rack-and-pinion steering apparatus, various proposals have been made on tooth specifications (pressure angle, module, number of teeth and so on) of the pinion teeth and the rack teeth (for example, refer to Japanese Examined Patent Application Laid-Open No. 62-38579 (1987)), for the purpose of maintaining a proper meshing state between the pinion teeth provided on the pinion shaft and the rack teeth provided on the rack shaft, to thereby offer a comfortable steering feeling to the driver.
For such rack-and-pinion steering apparatus, the tooth specifications on the pinion teeth provided on the pinion shaft are selected so as to satisfy design conditions provided by the vehicle on which the steering apparatus is to be mounted, more specifically to secure a strength that bears a load condition required on an arc of a length delimited by a travel amount of the rack shaft per rotation of the pinion shaft, i.e. a stroke ratio.
Here, the pressure angle included in the tooth specifications is, in most cases, determined according to the standard value (20° or 14.5°) stipulated by JIS (Japanese Industrial Standard) and, upon applying such standard pressure angle, other tooth specifications such as the module and the number of teeth, selected under an ordinary stroke ratio in a vehicle (35 to 60 mm/rev.), are determined as around 2.5 and 5, respectively.
In the rack-and-pinion steering apparatus, however, a special meshing form is employed, such as incorporating pressurizing means that utilizes a spring load for biasing the rack shaft with pressure toward the pinion shaft so as to mesh the pinion teeth and the rack teeth without backlash, in order to reduce rattling noise produced at the mesh portion between the pinion teeth and the rack teeth, thus preventing degradation of steering feeling.
When the tooth specifications for such pinion teeth are selected under the standard pressure angle, the pinion teeth are so deeply meshed because of the pressurization by the pressurizing means, the meshing friction with the rack teeth is increased, which restrains propagation of a reaction force from the road surface to the driver operating the steering member connected to the pinion shaft, thus resulting in degradation of the steering feeling, which leads to a drawback that it becomes difficult to execute micro adjustment of the steering operation when driving at a high speed on a low-μ road that provides only a small reaction force from the road surface and, therefore, to a disadvantage that the pressurizing load of the pressurizing means has to be strictly controlled.
Besides, the selected number of the pinion teeth is as few as 5 pieces, and therefore when such pinion teeth are meshed with the rack teeth, a phenomenon that the tip portion of a rack tooth that has passed the normal meshing position interferes with the dedendum of a pinion tooth as if gouging out the same, i.e. what is known as trochoid interference takes place, which further increases the meshing friction between the pinion teeth and rack teeth, thereby worsening the foregoing disadvantage. Moreover, when the trochoid interference severely takes place, the wall thickness of the dedendum of the pinion teeth is reduced over time by the friction with the tip portion of the rack teeth, which leads to degradation in strength of the pinion teeth, and even to premature breakdown before a desired life span is completed.
Further, in a column-assist type power steering apparatus provided with a motor for steering assistance located halfway of a column shaft connecting the steering member and the pinion shaft, for transmitting the rotational force of the motor to the pinion shaft via the column shaft thus to assist the steering operation, the pinion teeth provided on the pinion shaft are subjected to the rotating torque of the motor, in addition to the steering torque applied to the steering member by the driver. Therefore, the pinion teeth are more likely to breakdown, and besides the response of the rack shaft, upon transmitting thereto the rotation of the motor via the pinion shaft, becomes less perceptible, thus resulting in degradation in steering feeling.
Japanese Examined Patent Application Laid-Open No. 62-38579 (1987) only discloses a method of selecting a helix angle and a pressure angle of the rack teeth based on a relation with the cross-sectional shape of the rack shaft, for the purpose of preventing seizure between the pinion teeth and the rack teeth formed as skewed teeth, which occurs at the mesh portion therebetween because of the rotational displacement of the rack shaft around its axis, and hence does not provide any measures to solve the foregoing problems originating from the meshing friction and the trochoid interference.
Further, for preventing occurrence of the trochoid interference, conventionally the pinion teeth, as well as the rack teeth that mesh therewith, are formed as a “low tooth” having a tooth depth smaller than 1 module on an addendum side, compared with a pitch circle, to thereby secure a predetermined clearance. In this case, however, it becomes difficult to achieve a transverse contact ratio of 1 or greater between the rack teeth and the pinion teeth, and hence the meshing becomes discontinuous, thus creating another drawback that smooth movement of the rack shaft for the steering operation is disturbed.